With the advent of contagious diseases such as AIDS and Hepatitis B, the possibility of so called "needle stick" injuries in the medical environment has proliferated along with many and varied arrangements for preventing accidental infection of medical workers as a result of this type of injury. In addition, the proliferation of intravenous drug use has resulted in discarded syringes and needles on beaches and other areas where a passer-by can inadvertently step on a needle and also be infected.
As a result of this, there has been a proliferation of proposals for single use disposable syringes whereby a needle is retracted into the body of the syringe after a single use. However, most of these arrangements involve complete reworking of the conventional disposable syringe construction in such a way that the conventional luer lock disposable needle fitments are done away with such that the arrangements proposed are too complex and too expensive. Many also require additional manual steps to retract the needle.
A typical example of the prior art is U.S. Pat. No. 4,995,870 which describes a safety syringe where the needle bas a barb on the end inside the syringe for engaging the syringe plunger when the plunger is depressed to inject its contents into a patient. Once the barb engages the plunger the operator unlocks the needle and then withdraws the plunger to retract the needle into the syringe. The needle used is not a conventional luer lock fitment as it must have a barb on the end and the retraction process requires two manual steps which expose the operator of the syringe to the risk of "needle stick" injuries and are time-consuming and inconvenient.
A number of automatically retracting syringes have therefore been proposed. These include spring-loaded devices such as that described in U.S. Pat. No. 4,929,237 where a pushing force applied to the proximal end of the syringe moves the syringe within a housing against a spring to an operative position and releasing the pushing force allows the syringe to retract proximally drawing the needle back into the housing. The syringe is complex and likely to be expensive.
Australian Application No. 66,363/90 discloses a syringe where the plunger captures the needle and the plunger is withdrawn by a return spring when the downward pressure on it is released, thereby retracting the needle. The specification at page 6, line 22 refers to withdrawing the entire plunger and the attached needle back into the barrel of the syringe using a vacuum, but proposes no feasible way to do this. The only embodiments described use a return spring and the snap lock engagement means used to secure the needle to the plunger leaves a very large space (best seen in FIG. 3) in which medicament will remain when the plunger is withdrawn. The medicament is then likely to leak, creating a contamination hazard.
A vacuum-retractable syringe is proposed in Australian Application No. 70,672/91 where the needle is withdrawn into the interior of a hollow plunger under force of a vacuum within the plunger, however, in that invention the thrust exerted by the operator of the syringe must be sufficient to cause a washer at the end of travel of the plunger to rupture and to push a plug down through the washer to engage the needle. The needle mount (which is not a conventional luer lock) must also rupture so that the plug can withdraw the needle under vacuum into the plunger, where it is held in place by magnets. The syringe is complicated and unlikely to be feasible due to its complex operation.
These proposals have not found ready acceptance in the medical profession because of cost and inconvenience.